There is a need for an in-situ cell density monitor to provide continuous measurement of mammalian cell density inside of disposable, pre-sterilized bag bioreactors which have become an important new type of cell culture vessel for the production of monoclonal antibodies. Cell density is a key parameter for cell culture process diagnostic and optimization purposes. At present there is no available instrumentation for monitoring cell density in disposable bag bioreactors. The new optically based cell density measurement approach provides a simple and reliable method for in situ monitoring cell density inside of plastic bag bioreactors. The measurement approach can also be applied to other types of cell culture vessels including spinner flasks, T-flasks, and roller bottles that are also used grow mammalian cell cultures used for the production of monoclonal antibodies. The new cell density monitor will find applications in both research and production scale cell culture processes for producing monoclonal antibodies where the monitor will help increase the speed of process development and help optimize the process yields through improved process control.